Understanding Exposure – Part 3: Aperture

This is part three of our four part series, Understanding Exposure. This series of articles will explain everything a photographer needs to know about how light gets into their camera and how their camera uses that light to record the scene.

Inside the lens of your DSLR is a diaphragm that can open and close letting varying amounts of light pass through the lens to the sensor. This diaphragm and the size of it’s opening is often referred to as the aperture.

The small hole you can see in the center of the lens opens and closes to let varying amounts of light though to the image sensor.

The size of the diaphragm or aperture is given a number designation called f-stop. This number designation is counterintuitive in that the smaller the f-stop, the bigger or more wide open the diaphragm is.

So, an f-stop of 1.8 is larger and lets more light pass than an f-stop of 22.

I was going to explain what f-stop means technically but it’s really so over the top technical and won’t, in my opinion, help you take better pictures. If you want to know more about it, check out f-stop on Wikipedia.

Depth Of Field

The key thing about f-stops is depth of field — an easy way to remember it is the bigger the f-stop number, the more depth of field your photo will have. So an f-stop of 22 has much more depth of field than the same shot taken with an f-stop of 1.8.

From a creative perspective, a photographer can drastically alter his image by what f-stop he or she chooses to use. There may be times when you’ll want as much as possible in focus — perhaps in a sprawling landscape — in those cases you’d use a LARGE f-stop number to get a large depth of field which simply means that more of the image will be in focus.

Conversely, you may want a very shallow depth of field — often in portraiture we want the background out of focus. In that case we would choose a SMALL f-stop number to get a shallow depth of field.

In this photo I used an f-stop of 5.6. In it, you can see that the background is relatively blurred out.

In this photograph, I used an f-stop of 25. The larger number meant a larger depth of field hence the background is more in focus than the previous picture.

Lens Diffraction

To achieve maximum depth of field it may sound like a great idea to use the smallest aperture possible. The smallest aperture would mean that your f-stop number would be at the highest number available for your specific lens — remember, it’s counter-intuitive — the bigger the number, the smaller the aperture size. To use a smaller aperture is said to be Stopping Down. The smallest aperture on most lenses is F16 or F22 and if you shoot at the smallest aperture, you definitely will maximize the depth of field BUT, you’ll introduce image degradation into the picture called diffraction. Diffraction softens the image so overall, it won’t be as sharp as it could be. Most lenses are at their sharpest, and diffraction is at the minimum, when used with an aperture that is 2-4 stops closed down, from wide open… Ok, I’m likely getting you confused now so let’s take a deep breath, and step back and change the subject.

Stops

We know that all the so called f-stops let varying amounts of light through the lens. The smaller the number, the more light gets through. The bigger the f-stop number, the more light that will get through. The difference between F4 and F5.6 is said to be One (1) Full Stop. Between F4 and F8 is Two (2) full stops. Since the “F” numbers jump all over the place and aren’t in equal amounts like the ISO numbers are, it’s more difficult to know when your moving the aperture one, two, three or whatever full stops. That’s why we use a chart until we have all the stops memorized.

In the chart above, the full stops are the larger white numbers and the smaller black numbers below are 1/3 stops. Most lenses and cameras are capable of adjusting the aperture in 1/3 increments.

As you move through the full stops from left to right, you’re halving the amount of light hitting the sensor at each stop. So, F2 allows 1/2 the amount of light to pass than F1.4 does. To look at it another way, moving through the full stops from right to left doubles the amount of light hitting the sensor at each full stop. So, in other words, F1.4 lets in twice the amount of light pass than does F2.

Revisiting Lens Diffraction

I mentioned above that a lens is at it sharpest when it’s 2-4 stops closed down from wide open. To shoot your lens at it sharpest, take the widest aperture it has available — as an example, let’s say the widest the lens I’m using right now can open to is F2.8. Looking at our chart, 2-stops closed down from F2.8 is F5.6. Three stops is F8 and four stops is F11. So, to get the sharpest picture when using that specific lens, you’d like to shoot with an aperture between F5.6 and F11.

The lenses I use most each have a widest aperture of F2.8 and when I’m shooting a scene that requires a lot of depth of field, such as a landscape, I shoot with an aperture of F8. I’ve found that F8 gives me a super sharp picture and more than enough depth of field.

Aperture Priority

Most modern DSLR’s have a feature called Aperture Priority. All this means is that the photographer sets the aperture and the camera will adjust the shutter speed to properly expose the scene. Additionally, If the photographer has Auto ISO set, the camera will adjust the ISO as well.

Whenever you want to control the depth of field in a shot, chose to shoot in aperture priority and pick a small f-stop number for a shallow depth of field or a larger f-stop number for a greater depth of field.

That is the basics of aperture. In part four, the final part of this series, I’ll be discussing all three of the exposure triumvirate — ISO, Shutter Speed and Aperture and get a little more in depth of how they work together and offer real life situations where you would choose to give priority to one over the others.